linear hypothesis

[a.zapantis@qut.edu.au (Alex Zapantis)]

Dear Radsafers,

I see that once again the linear no threshold response hypothesis is being
challenged and that some subscribers seem to be absolutely convinced that
such an hypothesis has next to no scientific basis. In the past, various
studies have been cited as evidence of the invalidity of this hypothesis. 

I think that we should remind ourselves that based on the linear hypothesis
and the risk factors quoted by the ICRP, at low doses such as those which
are likely to occur at low dose rates, the risks are of such a magnitude
that studies which could disprove or prove (I use those words with some
licence) the linear hypothesis would require VERY large populations. eg, the
quoted risk of developing a fatal cancer from a dose of 1mSv (0.1rem) is 1
in 20,000. The "natural" rate of cancer mortality in the western world is
about 1 in 4.4. Thus even if we studied a population of 1E6 people, we would
be looking for 50 extra cases in about 227,000. I haven't even considered
confounding factors here. I guess what I am trying to say is that we
shouldn't be surprised if a study of 1000 people exposed to a few extra mSv
per year finds no excess incidence of cancer (or even a decreased risk!) .
Thus we certainly shouldn't use such studies as a basis for ridiculing the
linear model.

Our current knowledge of stochastic effects come largely from studies of
fairly large populations such as Nagasaki and Hiroshima, ankolosing
spondylitics (I can't spell it) etc, and based on ALL of this data, the ICRP
has presented the linear model. It may not be correct however I don't
believe that we have shown it to be erroneous at this stage. Remember, one
cannot prove a negative.

As professional HPs, I believe that it is our responsibility to provide
advice based on the current accepted state of knowledge. Those of us who
quote individual studies which include the words "radiation hormesis" in
their conclusions as a basis for discrediting the linear model remind me of
the lobbyists who use certain studies into ELF health effects (and ignore
the majority) to attempt to stop the erection of transmission lines in their
suburbs.

I do not propose that the linear model is the truth, the whole truth and
nothing but the truth, but I do believe that it is the best model we have
and that we should assume it to be correct until something better comes
along, which has not yet happened.

Well, I'm glad I got all of that off my chest. For all those who wish to
flame me, my direct email address is below.


Regards





                        Alex Zapantis
                        Radiation Safety Officer                               
                        Queensland University of Technology          
                        Health & Safety Section                             
                        Locked Bag No.2
                        Red Hill Qld 4059
                        AUSTRALIA

                        Ph     : 61 7 864 3566
                        fax     : 61 7 864 3993
                        email  : a.zapantis@qut.edu.au